Avian influenza patterns resemble 1918 pandemic, WHO study shows

Jul 3, 2006 (CIDRAP News) – The World Health Organization (WHO) on Friday published its first epidemiologic analysis of all laboratory-confirmed cases of H5N1 avian influenza reported to the agency between Dec 1, 2003, and Apr 30, 2006.

Though the quality, reliability, and format of the surveillance data varied by country, the WHO notes in its report that the conclusions are still useful for tracking disease patterns and planning future studies.

Over the time period studied, 205 cases were confirmed by polymerase chain reaction on respiratory tract specimens and/or microneutralization assay on serum specimens. Two asymptomatic Vietnamese patients whose cases were confirmed retrospectively from contact-screening samples were not included in the analysis, leaving a total of 203 cases, and the age of one patient was not known, leaving 202 cases available for age analyses.

The number of new countries reporting human cases of H5N1 has increased greatly in recent months as outbreaks among avian populations have spread, the study showed. From late 2003 through mid-2005, four countries reported human cases of H5N1 infection. In the much shorter period from October 2005 through the end of April 2006, five new countries reported cases in humans.

About half the cases occurred in people under age 20 (102 of 202), and 90% in people younger than age 40 (180 of 202). The high proportion of cases occurring in younger people may reflect the young population of the countries where the cases occurred, for example in Egypt and Indonesia. The WHO noted that the incomplete nature of the data makes it difficult to make reliable links between age and exposure and that more studies are needed to assess whether younger people or other groups are at increased risk of contracting the disease.

The overall fatality rate was 56% (113 of 202), with the incidence of death highest in those aged 10 to 39 years. The WHO emphasized that the fatality-rate patterns are notable for two reasons. They differ from the pattern of seasonal influenza, in which mortality is highest in elderly people. Also the case-fatality patterns resemble those observed during previous influenza pandemics—particularly the one that occurred in 1918.

The case-fatality rate was highest in 2004 (73%). It decreased to 43% in 2005, then has risen to 63% so far in 2006.

The researchers also examined the time intervals between symptom onset and hospitalization as well as between symptom onset and death and compared the intervals for different time periods. The median duration of illness onset to hospitalization for the entire surveillance period was 4 days and the median interval to death 9 days. There was no significant difference in the length of these intervals between 2004, 2005, and the first 4 months of 2006

Human cases have occurred year-round, but the peaks have coincided with winter and spring in the northern hemisphere. "If this pattern continues, an upsurge in cases could be anticipated starting in late 2006 or early 2007," the researches state.

The report highlights the gap in essential data needed to understand and refine treatment of patients with the disease. "Collecting more detailed information on antiviral treatments and outcomes, and particularly linking this information to sequential virological sampling, could inform future management decisions," the authors concluded.